1. Field of the Invention
The present invention relates to a mandrel for inserting in a fluid-flow borehole, the mandrel being of the type comprising: a body presenting a longitudinal axis; at least one radially-expandable annular assembly attached to the body, the annular assembly presenting a peripheral surface for pressing against a borehole and a peripheral surface for pressing against the body; and means for radially expanding the annular assembly; the annular assembly comprising, around the longitudinal axis, a plurality of radial expansion blocks, each block having a first bearing face defining a portion of the peripheral surface for pressing against the borehole, a second bearing face defining a portion of the peripheral surface for pressing against the body, and two side surfaces disposed respectively facing a side surface of at least one adjacent block.
The present invention applies in particular to securing a plug or a downhole tool in a leaktight manner in an oil well. The mandrel is suitable for use with wells provided with annular positioning grooves (also known as “landing nipples”) presenting a greater-diameter surface for providing sealing. The mandrel is also applicable to wells that do not include such regions.
The present invention also applies to scraping the inside walls of the well prior to securing a tool, and also to taking a molding of the inside walls.
2. Description of the Related Art
U.S. Pat. No. 6,695,051 discloses a mandrel of the above-specified type that comprises a tubular body, a radially-expandable annular sealing assembly, and deployable anchoring dogs (“slip segments”).
Such a mandrel is deployed in the well using a cable working line in order to form a plug in the well, e.g. when a deposit situated at the bottom of the well is no longer being worked. It may also be deployed using coiled tubing or using a string of drilling rods.
The annular sealing assembly comprises annular gaskets that can be expanded radially by applying compression. Such gaskets are built up from rubber rings so as to provide a sealing barrier around the mandrel in the borehole.
The rubber gaskets are clamped between top and bottom rings made of metal. When a seal is established around the mandrel, the top and bottom rings are moved towards each other, with the rubber situated between the rings being compressed, thereby causing it to expand radially.
Each ring is made up of a plurality of segments disposed in a staggered configuration so as to form an anti-extrusion barrier above and below the sealing gaskets. The segments spread apart from one another during radial expansion of the gaskets.
Such a mandrel is not entirely satisfactory. The expansion ratio of rubber gaskets is relatively small. In order to make the mandrel easier to lower down a well, it is necessary to leave sufficient space between the mandrel and the borehole, such that the rubber must be highly stressed in order to press in leaktight manner against the walls of the borehole, while nevertheless having an expansion ratio that is small in order to avoid the gasket suffering creep. Furthermore, if the mandrel is kept in the well for a long period of time, rubber gaskets tend to creep between the segments, and so they are subjected to irreversible deformation. It is then not possible to remove the mandrel easily from the well. It is then necessary to drill out the mandrel or to remove the completion, which leads to the mandrel being destroyed and to significant operating costs and/or production losses.